Field of the Invention
The present invention relates to a power generation control technique for a power generation unit mounted on a vehicle that is driven to travel by an electric motor.
Description of the Related Art
In electric vehicles that travel by driving an electric motor using power supplied from an on-vehicle battery, a vehicle incorporating a range extender serving as a power generation unit has been developed. The range extender is composed of, for example, a generator and a small engine dedicated to power generation. The generated power is supplied to the electric motor and used for charging the on-vehicle battery, thereby making it possible to increase the range of the electric vehicle.
Incidentally, progress has been made in the development of fuel cells recently and vehicles incorporating fuel cells have been proposed. Further, a vehicle using a fuel cell instead of an engine as a range extender for the electric vehicle has been proposed.
For example, Patent Document (Japanese Patent No. 5101583) discloses that in a vehicle incorporating a fuel cell, the fuel cell and a battery are used as a power supply source to supply power to an electric motor, so that drive wheels can be driven for travelling. Further, the Patent Document discloses a technique for controlling the output of the fuel cell so as to maintain a charging rate (SOC) of the battery at a target charging rate, which is set to a value close to a lower limit, after the power of the battery is used.
In the vehicle disclosed in the Patent Document described above, power is supplied from the battery to the electric motor until the battery charging rate reaches the target charging rate, and when the battery charging rate is decreased to a value lower than the target charging rate, the output from the fuel cell is controlled based on the difference between the present changing rate of the battery and the target charging rate thereof so that the power used for the electric motor and the like is provided and the target charging rate is maintained.
However, in general, when the output of a fuel cell increases, the efficiency of the output decreases. Even when the generator for driving the engine is used instead of the fuel cell, the range of an efficient rotational speed of the engine is limited. Accordingly, like in the Patent Document, when the power generation output of the power generation unit is set based on the charging rate and the target charging rate after the target charging rate is reached, the power generation unit need to output enough power for the vehicle power consumption, and when the power consumption of the vehicle is high, such as during high-speed travelling, it is necessary to increase the output from the fuel cell. This may deteriorate the efficiency of the power generation unit. Since the fuel of the power generation unit mounted on the vehicle is limited, the deterioration in the efficiency of the power generation unit results in a reduction in the range of the vehicle.